Scrap tires are generated at a rate of 270 million per year in the United States. They are non-biodegradable having been originally designed to last, when placed on a motorized vehicle, for tens of thousands of road miles. They are comprised of rubber, approximately 70% by weight, steel, approximately 20% by weight, and reinforcing textile fibers.
The disposal or reuse of previously used rubber products, such as rubber tires, presents many problems. Ecologically, rubber tires degrade very slowly and if disposed of improperly, may lead to hazardous environmental conditions in terms of both potential ground water problems and other ecological effects. The standard practice to remove these scrap tires from the solid waste stream has been to first run the tire through a tire shredder. Once shredded, the tire shreds can be returned to the solid waste stream, burned as tire derived fuel (TDF) or used as a feedstock for further processing into small, mesh size crumb rubber.
Recently, recycling of pre-used rubber products has increased in popularity in order to avoid potential negative environmental impact as well as to provide potentially commercially reusable rubber products.
Several methods for recycling used rubber products exist. Often, rubber products such as rubber tires are rendered into fine particulate rubber which then may be reused in other rubber products or other uses. However, one of the difficulties with recycling rubber products such as tires is that such products are extremely durable and consequently difficult to reduce to a re-usable form. In order for any recycling effort to be cost effective, a method must be developed to reduce the extremely durable rubber products to a form of rubber that may be useable in further generating processes. Recycled rubber particles become more commercially valuable with decreasing particle size. The commercial value increases because rubber particles of smaller sizes may be more easily incorporated into a wider variety of new rubber products or other applications.
Rubber recycling reclamation or granulating machines may be classified into two types. A first type produces fairly large rubber particles and operates at room temperature, relating to a shredder often using rotating knives to produce the particles. Although the process is fairly inexpensive, the large rubber particles produced, while usable for applications such as ground cover, are not generally usable for more commercially desirable applications such as new rubber products. A second type of rubber reclamation is cryogenic grinding methods where the machinery operates at extremely low temperatures using liquid nitrogen. Because the process requires continual replenishment of liquid nitrogen for maintaining operation at low temperatures, the process is fairly expensive.
Current two roll mill systems are used for a variety of purposes, including blending and mixing of elastomeric materials. Certain two roll mill designs have been used for reducing the particle size of materials. Two roll mills as previously designed have various drawbacks, such as the rolls are difficult to insert and remove for maintenance. This results in relatively long periods of down time, and subtracts from the productivity of the system in processing material. Another disadvantage with presently-known two roll mill systems is that the rolls are driven by electro-mechanical drives which must be designed to accommodate high shock loads, as encountered in rubber processing, for example. The high shock loads can ultimately lead to stalling of the shredder. Furthermore, unsealed bronze sleeve bearings typically used in two roll mill systems need to be water-cooled, and while handling high pressures and temperatures during size reduction, can be contaminated by the rubber particles entering the bearing. Current two roll mill systems also operate at a fixed friction ratio, and generally have no speed differential between the two rolls or, if any, only a ratio of up to 3:1 is obtained by gear reduction.
Accordingly, there is a need for a materials processing system that avoids the deficiencies of the prior art machines and methods, to provide a cost-effective and efficient processing system. Further, there is a need for an improved two roll mill system which overcomes the problems of previous mill designs for processing of materials, particularly elastomeric materials, having a fully variable friction ratio and sealed bearings capable of withstanding higher pressures and temperatures than current two roll mill systems.